1. Field of the Invention
The present invention relates to a process for the production of flame-resistant polyesters with at least 50 mol-% ethylene carboxylate units through co-polycondensation with at least one carboxyphosphinic acid.
2. Summary of the Related Art
Flame-resistant polyesters, especially for textile applications such as washable curtains or decorative materials, are preferably manufactured today by co-polycondensation with bi-functional phosphorus containing compounds. Carboxyphosphinic acids or their cyclic anhydrides, the phospholanes (U.S. Pat. No.3,941,752), are suited for that purpose. The disadvantage of the production of phospholanes is that it is relatively complex. The synthesis of carboxyphosphinic acids, for example of carboxyethyl(phenyl)phosphinic acid (IUPAC name 3-(phenyl(hydroxyphosphoryl))propanoic acid) (U.S. Pat. No. 4,081,463), is less complicated.
However, the carboxyphosphinic acids are bi-functional acids with a strong acidic nature (pH&lt;1.5). A result of this, in the case of its application as a co-monomer, the polyester produced therefrom has a higher content of diethylene glycol (DEG) with all of the accompanying disadvantages (e.g., stickiness of the granulate during drying and/or thermal pre-treatment before spinning, as well as a lowered melting temperature of the polyester to a point where it causes difficulties during ironing). During matting of the polyester with titanium dioxide, there is the risk that the titanium dioxide suspension will be rendered unstable by the acidic protons and that agglomerates will precipitate, leading to shorter filter clogging times and to difficulties during the spinning process. The acidic nature of the carboxyphosphinic acid has an especially negative effect on the batch production of polyester by direct esterification of dicarboxylic acids with ethylene glycol, because of the substantial amount of the esterification product of the previous batch remaining in the reactor, which causes the DEG content to rise from batch to batch.
It is also known that flame-resistant polyesters can be produced by co-polymerization with alkali salts of phosphinic acids (U.S. Pat. No. 3,900,444). Because of their mono-functional nature, however, such salts act as chain breaking agents and promote corrosion of the processing machinery. Others have used an alkali salt of a diolester of a bis-(carboxy)phosphinic acid as a co-monomer (DE Patent Specification 12 32 348). This co-monomer is produced by boiling bis-(carboxy)phosphinic acid and sodium chloride with an abundance of ethylene glycol for several hours. When sodium chloride is replaced with sodium carbonate, no salt is obtained. As a result of the then tri-functional nature of the bis-(carboxy)ester of the phosphinic acid, only heavily cross-linked, gel-like, unspinnable polyesters are produced.
It is further known that the formation of DEG during direct esterification of terephthalic acid with ethylene glycol and the subsequent polycondensation in the absence of carboxyphosphinic acid can be reduced by adding sodium or potassium hydroxide (U.S. Pat. No. 3,050,548), sodium oxide (DE-A 20 17 072), lithium or postassium hydroxide, lithium carbonate or sodium acetate (DE-A 1 520 976), or sodium or calcium acetate (U.S. Pat. No. 3,024,220) to the esterification reaction.
There are no suggestions with regard to the application and/or procedure during the co-polycondensation with carboxyphosphinic acids. In particular, there is no information regarding how the strongly acidic groups of the carboxyphosphinic acid, responsible for the higher DEG formation, can be neutralized without impairing their reactivity with regard to polyester formation. Consequently, new methods of producing flame-resistant polyesters are desired.